Your Prepaid plan is being phased out.
From 15 June 2016, we’re phasing out our Prepaid Cap, Pay As You Go, and 365 Day services. If you’re on one of these, don’t worry. You can stay on it for as long as you keep recharging, or you can switch to another Prepaid service.
Source:
http://www.vodafone.com.au/personal/mcp/payg-closure-opt-out
Converting people to 28 days plans no doubt